Conventional LTE (Long Term Evolution) cellular networks typically include a core network, a RAN (Radio Access Network), and one or more mobile communication devices (user equipment). The radio access network resides between user equipment (such as a mobile phone, a computer, or any remotely controlled machine) and provides connectivity with its core network (CN). As its name suggests, the radio access network provides respective user equipment access (such as Internet access) via the core network.
So-called 3rd Generation Partnership Project (3GPP) unites telecommunications standard development organizations (such as ARIB, ATIS, CCSA, ETSI, TSDSI, TTA, TTC), known as “Organizational Partners” and provides their members with a stable environment to produce the Reports and Specifications that define 3GPP technologies.
According to 3GPP TS 23.501 (Section 5.16.3), in order to support various deployment scenarios for obtaining IMS (IP Multimedia Subsystem) voice service, a serving PLMN (Public Land Mobile Network) over 3GPP access in 5GS (5G System) may provide IMS Voice over PS session in NR (New Radio) connected to 5GC (5G Core), via handover procedure to EPS (Evolved Packet System), via handover to E-UTRA connected to 5GC, or via redirection to EPS.
During the QoS Flow establishment procedure for IMS Voice, NG-RAN may trigger IMS Voice via handover or redirection procedure based on UE capabilities, N26 such as an interface between MME (Mobility Management Entity) and AMF (Access and Mobility Function) availability, network configuration and radio conditions. In the case of handover and redirection scenarios the IP (Internet Protocol) address of the UE (User Equipment) is preserved.
One of the triggering conditions for the NG-RAN to select the redirection to EPS (a.k.a., EPS fallback) procedure is when the interworking with N26 is not supported between the AMF (Access Mobility Function) and MME (Mobile Management Entity).
For this use case, during the 5G Registration of a UE, the serving PLMN (Public Land Mobility Network) AMF provides following relevant indications to the UE in Registration Accept message indicating that redirection to EPS is feasible for the registered UE in the Registration Area (as per Section 4.2.2.2.2 in 3GPP TS 23.502, Section 5.16.3.2 in 3GPP TS 23.501):
1) Network Support of Interworking without N26
2) IMS Voice over PS Session Supported Indication
Together with the indication above (i.e. Network Support of Interworking without N26 signaling), this indication means that UE supports redirection to EPS, EPS supports IMS voice, and NG-RAN supports redirection to EPS for this UE at QoS Flow establishment for IMS Voice.
For this purpose, as per the 3GPP specification, the AMF may need to perform the UE/RAN Radio information and Compatibility Request procedure during UE Registration to check the compatibility of the UE and RAN radio capabilities related to IMS Voice over PS.
Based on two indications above, the UE is aware of all the prerequisites required in the network (and the UE) that NG-RAN may redirect an IMS voice session to the E-UTRA in EPS.